Reflecting telescope

Stratospheric Observatory for Infrared Astronomy
24-inch convertible Newtonian/Cassegrain reflecting telescope on display at the Franklin Institute
A replica of Newton's second reflecting telescope that he presented to the Royal Society in 1672
The great telescope of Birr, the Leviathan of Parsonstown. Modern day remnants of the mirror and support structure.
Gran Telescopio Canarias
An image of Sirius A and Sirius B by the Hubble Space Telescope, showing diffraction spikes and concentric diffraction rings.
Main mirror of James Webb Space Telescope assembled at Goddard Space Flight Center, May 2016.
Light path in a Gregorian telescope.
Light path in a Newtonian telescope.
Light path in a Cassegrain telescope.
A prime focus telescope design. The observer/camera is at the focal point (shown as a red X).
Cassegrain design
Nasmyth/coudé light path.

Telescope that uses a single or a combination of curved mirrors that reflect light and form an image.

- Reflecting telescope

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Chromatic aberration

Failure of a lens to focus all colors to the same point.

Photographic example showing high quality lens (top) compared to lower quality model exhibiting transverse chromatic aberration (seen as a blur and a rainbow edge in areas of contrast.)
Chromatic correction of visible and near infrared wavelengths. Horizontal axis shows degree of aberration, 0 is no aberration. Lenses: 1: simple, 2: achromatic doublet, 3: apochromatic and 4: superachromat.
Color shifting through corner of eyeglasses.
Severe purple fringing can be seen at the edges of the horse's forelock, mane, and ear.
This photo taken with the lens aperture wide open resulting in a narrow depth-of-field and strong axial CA. The pendant has purple fringing in the near out-of-focus area and green fringing in the distance. Taken with a Nikon D7000 camera and an AF-S Nikkor 50mm f/1.8G lens.
Severe chromatic aberration

Isaac Newton's theories about white light being composed of a spectrum of colors led him to the conclusion that uneven refraction of light caused chromatic aberration (leading him to build the first reflecting telescope, his Newtonian telescope, in 1668.

Parabolic reflector

Reflective surface used to collect or project energy such as light, sound, or radio waves.

One of the world's largest solar parabolic dishes at the Ben-Gurion National Solar Energy Center in Israel
Circular paraboloid
Parallel rays coming into a parabolic mirror are focused at a point F. The vertex is V, and the axis of symmetry passes through V and F. For off-axis reflectors (with just the part of the paraboloid between the points P1 and P3), the receiver is still placed at the focus of the paraboloid, but it does not cast a shadow onto the reflector.
An oblique projection of a focus-balanced parabolic reflector
Antennas of the Atacama Large Millimeter Array on the Chajnantor Plateau
Lighting the Olympic Flame with a parabolic reflector
Off-axis satellite dish
The vertex of the paraboloid is below the bottom edge of the dish. The curvature of the dish is greatest near the vertex. The axis, which is aimed at the satellite, passes through the vertex and the receiver module, which is at the focus.

In optics, parabolic mirrors are used to gather light in reflecting telescopes and solar furnaces, and project a beam of light in flashlights, searchlights, stage spotlights, and car headlights.

Objective (optics)

Optical element that gathers light from the object being observed and focuses the light rays to produce a real image.

Several objective lenses on a microscope.
Objective lenses of binoculars
Two Leica oil immersion microscope objective lenses; left 100×, right 40×.
Camera photographic objective, focal length 50 mm, aperture 1:1.4
The segmented hexagonal objective mirror of the Keck 2 Telescope

In a telescope the objective is the lens at the front end of a refracting telescope (such as binoculars or telescopic sights) or the image-forming primary mirror of a reflecting or catadioptric telescope.

James Gregory (mathematician)

Scottish mathematician and astronomer.

James Gregory (1638–1675)
Vera circuli et hyperbolae quadratura, 1667
Diagram of a Gregorian reflecting telescope.

He described an early practical design for the reflecting telescope – the Gregorian telescope – and made advances in trigonometry, discovering infinite series representations for several trigonometric functions.

Curved mirror

Mirror with a curved reflecting surface.

Reflections in a convex mirror. The photographer is seen reflected at top right
A convex mirror diagram showing the focus, focal length, centre of curvature, principal axis, etc.
Convex mirror lets motorists see around a corner.
Detail of the convex mirror in the Arnolfini Portrait
A virtual image in a Christmas bauble.
A concave mirror diagram showing the focus, focal length, centre of curvature, principal axis, etc.

The most common non-spherical type are parabolic reflectors, found in optical devices such as reflecting telescopes that need to image distant objects, since spherical mirror systems, like spherical lenses, suffer from spherical aberration.


Optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects, or various devices used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation.

The 100-inch (2.54 m) Hooker reflecting telescope at Mount Wilson Observatory near Los Angeles, USA, used by Edwin Hubble to measure galaxy redshifts and discover the general expansion of the universe.
17th century telescope
The 60-inch Hale (debuted in 1908) considered to be the first modern large research reflecting telescope.
The primary mirror assembly of James Webb Space Telescope under construction. This is a segmented mirror and its coated with Gold to reflect (orange-red) visible light, through near-infrared to the mid-infrared
Modern telescopes typically use CCDs instead of film for recording images. This is the sensor array in the Kepler spacecraft.
A 1.2-meter (47 in) reflecting telescope
The Very Large Array at Socorro, New Mexico, United States.
Einstein Observatory was a space-based focusing optical X-ray telescope from 1978.
The Compton Gamma Ray Observatory is released into orbit by the Space Shutte in 1991, and it would operate until the year 2000
The reflectors of HEGRA detect flashes of light in the atmosphere, thus detecting high energy particles
Equatorial-mounted Keplerian telescope
A diagram of the electromagnetic spectrum with the Earth's atmospheric transmittance (or opacity) and the types of telescopes used to image parts of the spectrum.
Six views of the Crab nebula supernova remnant, viewed at different wavelengths of light by various telescopes
The Five-hundred-meter Aperture Spherical radio Telescope in Guizhou, China, is the world's largest filled-aperture radio telescope

The reflecting telescope, which uses mirrors to collect and focus light, was invented within a few decades of the first refracting telescope.

Newtonian telescope

Newtonian telescope design
Newtonian optical assembly showing the tube (1), the primary mirror (2), and the secondary diagonal mirror support (also called a "spider support") (3).
A large Newtonian reflector from 1873 with structure to access the eyepiece.
Newtonian Reflector
Very large trailer mounted Newtonian and its ladder
Newtonian (Truss-tube Dobsonian)
Altazimuth mounted Newtonian
Newtonian eyepiece mount
Amateur built 150mm Newtonian telescope
Astroscan, a commercial wide-field Newtonian reflector
Diagram of a commercial Newtonian reflector
Amateur commercial Newtonian diagram

The Newtonian telescope, also called the Newtonian reflector or just the Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton (1642–1727), using a concave primary mirror and a flat diagonal secondary mirror.

Hubble Space Telescope

Space telescope that was launched into low Earth orbit in 1990 and remains in operation.

Astronaut Owen Garriott working next to Skylab's crewed solar space observatory, 1973.
Dr. Nancy Grace Roman with a model of the Large Space Telescope that was eventually developed as the Hubble Space Telescope. While listed as a 1966 photo, this design was not the standard until the mid-1970s.
Grinding of Hubble's primary mirror at Perkin-Elmer, March 1979.
The backup mirror, by Kodak. Its inner support structure can be seen because it is not coated with a reflective surface.
The OTA, metering truss, and secondary baffle are visible in this image of Hubble during early construction.
DF-224 in Hubble, before it was replaced in 1999.
Exploded view of the Hubble Space Telescope
Hubble Control Center at Goddard Space Flight Center, 1999
STS-31 lifting off, carrying Hubble into orbit
Hubble being deployed from Discovery in 1990
An extract from a WF/PC image shows the light from a star spread over a wide area instead of being concentrated on a few pixels.
Optical evolution of Hubble's primary camera system. These images show spiral galaxy M100 as seen with WFPC1 in 1993 before corrective optics (left), with WFPC2 in 1994 after correction (center), and with WFC3 in 2018 (right).
COSTAR being removed in 2009
Astronauts Musgrave and Hoffman install corrective optics during SM1
Hubble as seen from Discovery during its second servicing mission
Nickel–hydrogen battery pack for Hubble
One of Hubble's most famous images, Pillars of Creation, shows stars forming in the Eagle Nebula.
The Frontier Fields program studied MACS0416.1-2403.
Star cluster Pismis 24 with nebula
The HST is sometimes visible from the ground, as in this 39-second exposure when it is in Orion. Maximum brightness about magnitude 1.
Hubble Extreme Deep Field image of space in the constellation Fornax
An illustration of a black hole
Hubble's STIS UV and ACS visible light combined to reveal Saturn's southern aurora
Brown spots mark Comet Shoemaker–Levy 9 impact sites on Jupiter's southern hemisphere. Imaged by Hubble.
Hubble and ALMA image of MACS J1149.5+2223
Evolution of detecting the early Universe
Some of the Carina nebula by WFC3
Hubble precision stellar distance measurement has been extended ten times further into the Milky Way.
Data analysis of a spectrum revealing the chemistry of hidden clouds
In 2001, NASA polled internet users to find out what they would most like Hubble to observe; they overwhelmingly selected the Horsehead Nebula.
One-quarter scale model at the courthouse in Marshfield, Missouri, a hometown of Edwin Hubble
A pillar of gas and dust in the Carina Nebula. This Wide Field Camera 3 image, dubbed Mystic Mountain, was released in 2010 to commemorate Hubble's 20th anniversary in space.
Hubble views the Fomalhaut system. This false-color image was taken in October 2004 and July 2006 with the Advanced Camera for Surveys.
Illustration of the Soft Capture Mechanism (SCM) installed on Hubble
Hubble and JWST mirrors (4.0 m2 and 25 m2 respectively)

In 1968, NASA developed firm plans for a space-based reflecting telescope with a mirror 3 m in diameter, known provisionally as the Large Orbiting Telescope or Large Space Telescope (LST), with a launch slated for 1979.

Primary mirror

The largest non-segmented mirror in an optical telescope in 2009, one of the Large Binocular Telescope's two mirrors
Six of the 18 primary mirrors of the James Webb Space Telescope being prepared for acceptance testing
The correctly ground backup primary mirror built by Eastman Kodak for the Hubble space telescope (the mirror was never coated with a reflective surface, hence its honeycomb support structure is visible). It now resides in the National Air and Space Museum in Washington, DC.
Artists's Rendering of the primary mirror of the ELT.

A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope.

Refracting telescope

Type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope).

A 200 mm refracting telescope at the Poznań Observatory
Optical diagram of Galilean telescope
y – Distant object; y′ – Real image from objective; y″ – Magnified virtual image from eyepiece;
D – Entrance pupil diameter; d – Virtual exit pupil diameter;  L1 – Objective lens;  L2 – Eyepiece lens e – Virtual exit pupil – Telescope equals
Engraved illustration of a 150 ft focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.
Alvan Clark polishes the big Yerkes achromatic objective lens, over 1 meter across, in 1896.
This 12 inch refractor is mounted in dome and a mount the rotates with the turn of the Earth
The Greenwich 28-inch refractor is a popular tourist attraction in 21st century London
The Apochromatic lens usually comprises three elements that bring light of three different frequencies to a common focus
The 102 cm refractor, at Yerkes Observatory, the largest achromatic refractor ever put into astronomical use (photo taken on 6 May 1921, as Einstein was visiting)
The "Große Refraktor" a double telescope with a 80cm (31.5") and 50 cm (19.5") lenses, was used to discover calcium as an interstellar medium in 1904.
Astronaut trains with camera with large lens
Touristic telescope pointed to Matterhorn in Switzerland
The Yerkes Great refractor mounted at the 1893 World's Fair in Chicago; the tallest, longest, and biggest aperture refactor up to that time.
The 68 cm refractor at the Vienna University Observatory

Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by the reflecting telescope, which allows larger apertures.